I am trying to design a larger hub and had some ideas, my first pass is using cheap lazy susan bearings, but it doesn't feel ideal and they have a fair bit of slop in them.

I was thinking that since none of the arms need to continuous rotation around the entire hub, they could actually share the same rail. as in, have two different carriers on the same circular rail, something like this http://img.directindustry.com/images_di ... 943423.jpg

So, one arm will terminate in a fixed annulus of 120mm or so diameter, and each of the other arms would terminate in little cars that clamp onto the annulus via cheap v-groove bearings and travel freely along it.

I am not quite sure how well a printed annulus will work though in practice, I know a few deltabots use printed linear cars that use bearings so i am fairly confident that will work. Maybe a metal ring that can stand up to bearings can be sourced easily?

This is a really cool Idea! The strength in this type of design is going to come from the distance between the two bearings attached to the rail. I might suggest using two rails aligned vertically that way you can achieve strength from the vertical distance between these two rails instead of horizontal distance between two bearings along the rail. See my attached low-quality sketches.

Sourcing cheap metal rings to run is probably your best bet. My guess would be it shouldn't be too difficult, just need to figure out the right terms to search... can't think of them off the top of my head.